This invention relates to a method of fabricating uranium dioxide powder useful as nuclear fuel by conversion of uranium / hexafluoride, more particularly an improvement in ammonium diuranate (ADU) method.
Making a general classification of methods of converting uranium hexafluoride (UF.sub.6) into uranium dioxide (UO.sub.2) powder useful as nuclear fuel, there are two types of methods, i.e., wet methods and dry methods. Representative examples of the wet methods include a so-called ammonium diuranate (ADU) method in which UF.sub.6 gas is reacted with water to form an aqueous uranyl fluoride (UO.sub.2 F.sub.2) solution, which is then reacted with ammonia to precipitate ammonium diuranate (ADU) and the precipitate is subjected to various steps such as filtration, drying, calcination, reduction, etc. to obtain UO.sub.2 powder.
Ammonium diuranate is prepared according to the following reaction schemes (I) and (II). EQU UF.sub.6 +2H.sub.2 O.fwdarw.UO.sub.2 F.sub.2 +4HF (I) EQU UO.sub.2 F.sub.2 +4HF+7NH.sub.3 +7H.sub.2 O.fwdarw.1/2(NH.sub.4).sub.2 U.sub.2 O.sub.7 +6NH.sub.4 F+11/2H.sub.2 O (II)
In the above-described method, there are four moles of hydrogen fluoride (HF) per mole of uranium (U) in the aqueous UO.sub.2 F.sub.2 solution formed by the hydrolysis of UF.sub.6 as shown in formula (I) above. This is disadvantageous in two respects. In the first instance, it is difficult to obtain UO.sub.2 powder having the high activity by employing the above-described method.
On the first point, in the formation reaction of ADU represented by reaction scheme (II), firstly ammonium fluoride (NH.sub.4 F) is formed by neutralization reaction with HF, resulting in that primary grains of ADU become relatively large. UO.sub.2 powder obtained by subjecting ADU composed of large primary grains to various steps such as calcination, reduction, etc. is composed of large primary grains as in the case of ADU. This means that the activity of the powder is relatively low. When sintered pellets, i.e., nuclear fuel elements, are produced from such UO.sub.2 powder having a relatively low activity, the sintered density of pellets produced under ordinary conditions is about 95% TD (theoretical density) and the pellets are composed of crystals having a grain size of an order of 10 micrometers. When it is desired to obtain pellets with sintered density and grain size greater than those described above, attempts to carry out sintering at elevated temperatures or for a prolonged period of time would be successful to some extent although it is practically difficult to obtain pellets with a grain size of 40 to 50 micrometers, for example.
On the second point, fluorine (F) is at first converted into ammonium fluoride (NH.sub.4 F) and is contained in the filtrate after filtering off ADU precipitate. The liquid waste which contains NH.sub.4 F is treated with a precipitant such as slaked lime (Ca(OH).sub.2). With this treatment fluorine component is precipitated as calcium fluoride (CaF.sub.2) and ammonia (NH.sub.3) is recovered and recycled. On the other hand, CaF.sub.2, which is formed in a very large amount e.g., as large as about 1 ton per ton of uranium treated, is left as solid waste without being further recycled. In the conventional ADU methods, it has been difficult to obtain UO.sub.2 powder having a high activity due to adverse influence of fluorine, and in addition they are disadvantageous in that a large amount of fluorides are formed as wastes.
Examples of wet methods include, besides the abovedescribed ADU method, an ammonium uranyl carbonate (AUC) method, a modified ADU method, etc. The AUC method is a method in which UF.sub.6 gas is reacted with ammonia, carbon dioxide and water to precipitate ammonium uranyl carbonate (AUC) and then subjected to steps of filtration, drying, calcination, reduction, etc. substantially in the same manner as in the ADU method to obtain UO.sub.2 powder. On the other hand, the modified ADU method comprises hydrolyzing UF.sub.6 gas in a nitric acid solution containing a defluorinating agent to obtain an aqueous UO.sub.2 (NO.sub.3)2, purifying the solution by solvent extraction, reacting the purified solution with ammonia to form ADU and subjecting the resulting ADU to the same treatments as in the ADU method, thus converting the ADU into UO.sub.2. Although these methods enable fabrication of UO.sub.2 powder having activity higher than that of the powder obtained according to ordinary ADU method, they not only fail to give rise to UO.sub.2 powder with a grain size of larger than about 20 micrometers but also they suffer substantially the same disadvantage as encountered in the ordinary ADU method with respect to the occurrence of fluoride wastes. More particularly, in the AUC method fluorine remains in the final product as CaF.sub.2 as in the ADU method while in the modified ADU method fluorides of the defluorinating agent, e.g., aluminum fluoride when aluminum nitrate is used as a defluorinating agent, are found in the final product.
The dry method, which is excellent since it is free of the above-described defects such as formation of a large amount of fluoride waste in the wet ADU method, is represented by the following reaction schemes. EQU UF.sub.6 +2H.sub.2 O.fwdarw.UO.sub.2 F.sub.2 +4HF (III) EQU UO.sub.2 F.sub.2 +H.sub.2 .fwdarw.UO.sub.2 +2HF (IV)
In the above reactions, at first UF.sub.6 is reacted with steam to form UO.sub.2 F.sub.2 which is solid, with HF being separated as gas therefrom. Then, UO.sub.2 F.sub.2 is reduced with hydrogen gas to give rise to UO.sub.2 powder. In this method, all the fluorine component is recovered as HF gas in contrast to the wet method in which the fluorine component goes to liquid waste which causes the occurrence of fluoride wastes. However, according to the dry method, it is generally difficult to control the characteristics of UO.sub.2 to be obtained and only UO.sub.2 powder having activity lower than that of the powder from wet ADU method is obtained.